In their early development neonatal and fetal animals acquire passive immunity as a consequence of the selective transport of maternal immunoglobulin G (IgG) from maternal serum to the circulation of the young. Other immunoglobulins do not traverse the tissue forming the maternofetal interface; the process is specific for IgG. The yolk sac in fowl and rabbits, the gut in many rodents, and the placenta in most other mammals are the primary selective barriers between mother and young. In the oocyte, it is postulated that IgG binds to specific receptors localized to the plasma membrane of the apical surface of the epithelial cells. We intend to investigate the parameters of the immunoglobulin binding to chicken oocyte plasma membranes. In particular, optimal conditions for binding, kinetic parameters, portion of IgG responsible for binding, enzymic sensitivity of the receptor and the morphological localization of IgG binging will be examined.